Add a time to blocks about catena

[pixelspark]

So how would we deal with the timestamp for the genesis block? We use the seed to calculate the genesis block, but the genesis block changes if it includes a timestamp. The options are:

• Have no timestamp in the genesis block (special-case it)
• Have a fixed timestamp in the genesis block (01-01-1970..?). This leads to issues with the median calcuation for the first x blocks
• Have an actual timestamp in the genesis block and (1) provide it when bootstrapping together with the seed string, or (2) change bootstrapping to use the genesis signature instead (and fetch the genesis block from a peer).

[pixelspark]

Should also expose the average time between the last N blocks ($movingAverageBlockTime?) so it can be used to regulate difficulty (see #36)